Hair Cell Transduction Research Articles

A dual filter model describing single-fiber responses to clicks in the normal and noise-damaged cochlea.

This paper presents a composite model of the normal and noise-damaged guinea pig cochlea. The model incorporates a phenomenologically defined cochlear filter, and physiological descriptions of inner hair cell transduction, synaptic adaptation, and spike generation. The latter three model sections were taken from recent literature. The paper first deals with validation and evaluation of the model and adaptation of the relevant parameters to the guinea pig. Then the model is applied to explore to what extent changes in the cochlear filter can be held responsible for abnormal responses to clicks that were recorded in single auditory nerve fibers in noise-damaged animals. Focus is on those fibers in which the tip-to-tail sensitivity ratio of the frequency threshold curve (FTC) has decreased and/or in which the FTC tail has become hypersensitive. Inspired by this type of W-shaped FTC the mechanical response of the basilar membrane is phenomenologically modeled by two parallel filters, one responsible for the tip of the FTC, the other for its tail. Model simulations show that most abnormal temporal response properties can be explained by pathological alterations in the mechanical response. Residual discrepancies between model and experiment are identified which presumably point to pathological changes in other stages of cochlear processing.

Block by amiloride and its derivatives of mechano-electrical transduction in outer hair cells of mouse cochlear cultures.

1. The effects of amiloride and amiloride derivatives on mechano-electrical transducer currents in outer hair cells of the cultured neonatal mouse cochlea were examined under whole-cell voltage clamp. 2. At -84 mV transducer currents were reversibly blocked by the extracellular application of the pyrazinecarboxamides amiloride, benzamil, dimethylamiloride, hexamethyleneiminoamiloride, phenamil and methoxynitroiodobenzamil with half-blocking concentrations of 53, 5.5, 40, 4.3, 12 and 1.8 microM, respectively. Hill coefficients were determined for all but the last of these compounds and were 1.7, 1.6, 1.0, 2.2 and 1.6, respectively, suggesting that two drug molecules co-operatively block the transducer channel. 3. Both the structure-activity sequence for amiloride and its derivatives and the mechanism of the block of the transducer channel appear to be different from those reported for the high-affinity amiloride-sensitive epithelial Na+ channels but similar to those of stretch-activated channels in Xenopus oocytes. 4. The block by all pyrazinecarboxamides was voltage dependent with positive membrane potentials releasing the block. The form of the voltage dependence is consistent with a voltage-independent binding of the drug to a site that is accessible at hyperpolarized but not at depolarized potentials, suggesting that the transducer channel undergoes a voltage-dependent conformational change. The channel was not blocked by 1 mM amiloride from the intracellular side at either negative or positive membrane potentials. 5. The kinetics of the block were studied using force steps or voltage jumps. The results suggest that the drug binding site is only accessible when the transducer channel is open (open-channel block) and that the channel cannot close when the drug molecules are bound. 6. The time dependence and voltage dependence of the block together reveal that the transducer channel has at least two open conformational states, the transition between which is voltage dependent.

Nonlinear terms produced by passing amplitude-modulated sinusoids through a hair cell transducer function

We derive mathematically the output of Corey and Hudspeth's hair cell transducer function for the following cases: (1) the input is a single carrier whose output is modulated by the sum of two sinusoids; (2) the input is the sum of two carriers, each of which is amplitude-modulated by a single sinusoid. The theoretical results are similar to the results of an experiment in which field potentials were recorded from the human scalp while one ear was stimulated with auditory waveform (1) and with auditory waveform (2) of the cases above.

Nonlinear terms produced by passing amplitude-modulated sinusoids through a hair cell transducer function

Nonlinear terms produced by passing amplitude-modulated sinusoids through a hair cell transducer function

A mathematical model of hair cell transduction

A mathematical model is described for mechano-electro-chemical transduction in hair cells. The model relates the displacement of stereocilia to the intensity of firing of fibers attached to each hair cell through a cascade of four stages. The gatingstage relates hair bundle displacement to the change in conductivity of the hair cell membrane. It is described by a mechanical version of the Hodgkin–Huxley model. The electricstage relates the change in conductivity to the receptor potential through a hair cell electrical network of the Davis type. The calciumkineticsstage relates free calcium ion (Ca2+) concentration in hair cells to the receptor potential. It is assumed there are two states of calcium ions: free and bound (by buffers). The synaptictransmissionstage relates free calcium concentration to the intensity of firing as described by the Hill equation. Using relevant parameters, the model simulates the behavior of low- and high-spontaneous firing rate fibers, and is being implemented as a component of a complete cochlear model. [Work supported by NSERC, URIF, and Unitron.]

Adenine nucleoside diphosphates block adaptation of mechanoelectrical transduction in hair cells.

By adapting to sustained stimuli, hair cells in the internal ear retain their sensitivity to minute transient displacements. Because one model for adaptation asserts that this process is mediated by a myosin isozyme, we reasoned that we should be able to arrest adaptation by interfering with myosin's ATPase cycle though introduction of ADP into hair cells. During tight-seal, whole-cell recordings of transduction currents in cells isolated from bullfrog (Rana catesbeiana) sacculus, dialysis with 5-25 mM ADP gave variable results. In half of the cells examined, the rate of adaptation remained unchanged or even increased; adaptation was blocked in the remaining cells. Because we suspected that the variable effect of ADP resulted from the conversion of ADP to ATP by adenylate kinase, we employed the ADP analog adenosine 5'-[beta-thio]diphosphate (ADP[beta S]), which is not a substrate for adenylate kinase. Adaptation consistently disappeared in the presence of 1-10 mM ADP[beta S]; in addition, the transduction channels' open probability at rest grew from approximately 0.1 to 0.8 or more. Both effects could be reversed by 2 mM ATP. When used in conjunction with the adenylate kinase inhibitor P1,P5-bis(5'-adenosyl) pentaphosphate (Ap5A), ADP had effects similar to those of ADP[beta S]. These results suggest that adaptation by hair cells involves adenine nucleotides, and they lend support to the hypothesis that the adaptation process is powered by a myosin motor.

Modeling vowel identification by listeners: A comparison of cepstrum-based and auditory models at reduced signal-to-noise ratios

This study compared the abilities of two computational models to predict the identification responses of listeners to vowels presented in quiet and with pink-noise maskers at signal-to-noise ratios (SNRs) of −6, −3, 0, +3, and +6 dB. One model used a representation derived from cepstral analysis. The other used a representation derived from a model of peripheral auditory analysis which (i) filtered the stimulus with a bank of bandpass filters; (ii) compressed the filtered waveforms using a simulation of hair cell transduction; (iii) computed the short-term autocorrelation function of the compressed, filtered waveform in each channel; and (iv) summed the autocorrelation functions across filter channels to generate a pooled autocorrelation function (PACF). Classification was performed by discriminant function analysis, using noise-free tokens as input to the classifier during the training phase. Performance using the cepstrum-based model was better than human listeners in quiet, but deteriorated rapidly with the audition of background noise. In contrast, the PACF-based model performed at levels similar to listeners in quiet, and mirrored their gradual decline in accuracy with decreasing SNR.

Nonlinear mechanical responses of mouse cochlear hair bundles.

The stiffness of sensory hair bundles of both inner (IHC) and outer (OHC) hair cells was measured with calibrated silica fibres in mouse cochlear cultures to test the hypothesis that the mechanical properties of the hair bundle reflect processes underlying mechanotransduction. For OHCs, the displacement of the hair bundle relaxed with time constants of 6 ms for displacements which open transducer channels and 4 ms for displacements which close the channels. The corresponding values of the time constants for IHCs were 10 ms and 8 ms, respectively. A displacement-dependent change in the stiffness of the hair bundle was not observed when the bundle was displaced orthogonally to the direction of excitation. The stiffness of the hair bundle as a function of nanometre displacements from the resting position was remarkably nonlinear. The stiffness declined to a minimum from the resting stiffness by about 12% for OHCs and 20% for IHCs when the hair bundle was displaced by about 20 nm in the excitatory direction, and it increased by a similar amount when the bundle was displaced by 20 nm in the inhibitory direction. The displacement at which the stiffness reached a minimum was within the most sensitive region of the hair-cell transducer function (receptor potential as a function of hair-bundle displacement), and the displacement at which the stiffness reached a maximum was at the point of saturation of the transducer function in the inhibitory direction. The nonlinear displacement-dependent compliance change is reversibly abolished, and the time constant of relaxation of the bundle for excitatory displacements is reversibly reduced, when mechanotransduction is blocked by the addition of either neomycin sulphate or cobalt chloride to the solution bathing the hair cells. The displacement-dependent compliance change was not apparently reduced when the receptor potential was attenuated through the substitution of sodium in the bathing solution with a less permeant cation, tetraethylammonium. These findings suggest that the nonlinear mechanical properties of the hair bundle are associated with aspects of the hair-cell mechanotransducer process. The mechanical properties of the hair bundle are discussed in relation to the 'gating-spring' hypothesis of hair-cell transduction.

Additivity of threshold elevations produced by disruption of outer hair cell function

We present a simple model describing the additivity of hearing loss in the mammalian cochlea produced by disruption of the outer hair cell transduction processes. The validity of this model has been tested experimentally in the guinea-pig by inducing threshold elevations using two simultaneous cochlear manipulations, including acoustic overstimulation, two-tone suppression, low-frequency acoustic biasing of the cochlear partition and electrical stimulation of the medial olivo-cochlear system of efferent fibres. The results of these experiments suggest that the model presented is an adequate description, within the measurement error of our experiments, of the hearing losses produced.

Putative immunolocalization of the mechanoelectrical transduction channels in mammalian cochlear hair cells.

Hair cells bear an apical bundle of stereocilia arranged in serried rows. Deflection of the bundle controls the opening and closing of mechanoelectrical transduction channels, thereby altering the conductance across the apical plasma membrane. Two locations for these channels have been proposed in the bundle, either near the bases of the stereocilia or towards their tips. One hypothesis that is consistent with the latter possibility suggests that fine extracellular filaments, which run between the tips of the shorter stereocilia and the sides of the taller stereocilia behind, operate the channels. Determining the precise position of the channels is essential to test this hypothesis. We have therefore attempted to localize them immunocytochemically. Because hair-cell transduction is amiloride sensitive, the channels may have an amiloride-binding site associated with them. We have therefore used a polyclonal antibody raised against another amiloride-sensitive ion channel to hunt for them. This antibody recognizes a 62-64 kDa band in immunoblots of cochlear tissue, and produces discrete labelling in the hair bundle. This is most concentrated just below the tips of the shorter stereocilia, coinciding with a region of specialization in the closely apposed membranes of the short and tall stereocilia but not with either end of the tip link.

Mechanical preprocessing in the mammalian cochlea.

The mammalian cochlea responds with exquisite sensitivity to the small fluctuations in air pressure that make up the stimulus of sound. Moreover, it responds to pressure fluctuations that occur extremely rapidly and that vary over a wide range of intensities--in both cases, to an extent outside the capabilities of unaided nerve fibres. Research performed during the past decade has shown that these properties are dependent on a physiological source of mechanical energy that operates probably within the outer hair cells of the organ of Corti. These cells, which are anatomically and functionally similar to the primary receptor cells, the inner hair cells, are believed to function as a source of mechanical power to assist the mechanical sensitivity of the cochlea, by mechanisms that currently are not understood. Several possible mechanisms have been proposed, but each has limitations that may make it an unsuitable candidate. Recent work has also demonstrated the likely role of mechanoelectrical transduction in outer hair cells in controlling the power source and thereby influencing the sensitivity and amplitude range of the cochlea.

Tip-link integrity and mechanical transduction in vertebrate hair cells

An attractive hypothesis for hair-cell transduction is that fine, filamentous “tip links” pull directly on mechanically sensitive ion channels located at the tips of the stereocilia. We tested the involvement of tip links in the transduction process by treating bundles with a BAPTAbuffered, low-Ca 2+ saline (10 −9 M). BAPTA abolished the transduction current in a few hundred milliseconds. BAPTA treatment for a few seconds eliminated the tip links observed by either scanning or transmission electron microscopy. BAPTA also eliminated the voltagedependent movement and caused a positive bundle displacement of 133 nm, in quantitative agreement with a model for regulation of tension. We conclude that tip links convey tension to the transduction channels of hair cells.

Monoclonal antibodies to inner ear antigens: II Antigens expressed in sensory cell stereocilia

To develop biological reagents for investigating structure-function relationships in the organ of Corti, we have raised monoclonal antibodies, (MAb) to inner ear tissues. Our first series of antibodies prepared after intrasplenic immunization of mice with guinea pig tissues, identified antigens restricted to supporting cell structures, but no hair cell specific antibodies were developed [Zajic et al., Hear. Res. 52, 59–72, 1991]. In this report we describe the isolation, binding specificity and initial characterization of the stereocilia-binding monoclonal antibodies, KHRI-4, and KHRI-5. Mice were immunized with avian, amphibian and mammalian sensory hair cell-containing tissues and antibodies were screened for selective binding to cochlear extracts in ELISA. In the inner ear, KHRI-4 and KHRI-5 bind specifically to stereocilia in both avian and mammalian cochlear and vestibular tissue preparations using immunofluorescence and immunoperoxidase assays. In other tissues only certain cells of mesothelial origin, such as smooth muscle in gut and the arteriolar vasculature, were stained by KHRI-4 indicating that the antigenic structure defined by this antibody has limited distribution. KHRI-5 binding could be detected in other tissues only at high antibody concentrations suggesting that the gene product identified by this antibody is also weakly expressed in other cell lineages. Western blot analysis showed that KHRI-4 and -5 detect different protein complexes. KHRI-4 identifies an antigenic structure common to gut, cochlea, vestibular tissue and cultured fibroblasts consisting of a ~ 195 and a 230 kDa heterodimer designated p195 230 . KHRI-5 binds to a prominent ~ 200–210 kDa band in Western blots of cochlear tissues, gut and fibroblasts. In immunoprecipitation experiments, KHRI-5 precipitated three proteins of M r ~ 200–210, 230 and 260 kDa indicating that the ~ 200–210 kDa protein carrying the epitope for this antibody is a member of a heterotrimer complex. Our results show that these protein complexes are structural components of stereocilia and that the same proteins are arrayed in conjunction with the actin stress fibers of cultured mesothelial cells. Thus, they are likely to be important for maintaining the actin structure of stereocilia essential to transduction in sensory hair cells.

Modeling Normal and Impaired Hearing

A cochlear simulation has been developed to model normal and impaired hearing. The simulation includes the middle ear, the mechanical motion of the cochlear partition, and the mechanical to neural transduction of the inner hair cells. The outer hair cells are postulated to provide an active feedback mechanism that adjusts the gain and shape of the auditory filters. Auditory impairment is simulated by reducing the efficacy of the outer hair cells and by modifying the inner hair cell transduction process. The effects of simulated impairment are illustrated for two speech sounds, /ba/ and /ka/, with the neural firing patterns from the impaired ear compared with those of a normal ear. The differences in the neural firing patterns are interpreted in the context of hearing aid signal processing.

Localization of the hair cell's transduction channels at the hair bundle's top by iontophoretic application of a channel blocker

In order to understand how the hair cell's mechanoelectrical transduction channels are gated during mechanical stimulation, it is essential to determine their location with respect to the hair bundle's constituent stereocilia. We localized the transduction channels by focally blocking receptor currents with iontophoretically ejected gentamicin, an aminoglycoside antibiotic that acts as a reversible channel blocker. The drug was most effective when directed at the top of a hair bundle, whereas application at the bundle's bottom or at the cuticular plate had little or no effect. Computer simulations of blocking agreed with experimental data only when the transduction channels were hypothesized to occur near the bundle's top. These results confirm that the hair cell's transduction channels are located near the stereociliary tips.

Cochlear efferent neurones and protection against acoustic trauma: Protection of outer hair cell receptor current and interanimal variability

We have measured the changes in neural and microphonic sensitivity in the basal turn of the guinea-pig cochlea produced by intense acoustic overstimulation (10 kHz, 115 dB SPL for 60 s and 150 s). As reported previously, the drop in neural and microphonic sensitivities observed after overstimulation were highly correlated [Patuzzi et al. (1989) Hear. Res. 39, 189–202]. Presentation of a non-traumatizing pure-tone to the contralateral ear (10 kHz, 80 dB SPL) during acoustic overstimulation reduced the amount of acoustic trauma measured using the neural response or the microphonic response. Transection of the medial olivo-cochlear system of efferent fibres at the floor of the fourth ventricle abolished this protective effect of contralateral sound and dramatically reduced the variability in the data. Since the low-frequency microphonic is a simple measure of the receptor current through the outer hair cells, and this current probably plays a part in enhancing the mechanical sensitivity of the cochlea, the protection of the microphonic we have observed suggests that the efferent system protects neural sensitivity by protecting the mechano-electrical transduction of outer hair cells. The drop in variability after sectioning the efferents also suggests that inter-animal variations in susceptibility to noise trauma may be a consequence of differing tonic activity of the efferents, and/or a variation in the sensitivity of the efferent pathway.

A time-domain digital cochlear model

The author presents a digital time-domain model of the human cochlea designed to represent normal auditory functioning and to allow for degradation related to auditory impairment. The model consists of the middle ear, the mechanical motion of the cochlea, and the neural transduction of the inner hair cells. The traveling waves on the cochlear partition are represented by a cascade of digital filter sections, and the cochlear micromechanics are represented by a second filter that further sharpens the excitation to the inner hair cells. The neural firing rate is determined by the sum of the outputs of multiple fibers attached to each inner hair cell, with the fiber neurons having firing characteristics representative of low- and high-spontaneous-rate fibers. The signal processing cochlear model incorporates dynamic-range compression by adjusting the Q of each cochlear filter section and second filter in response to the second-filter velocity and the averaged neural firing rate. Examples of the model response to impulse and tone-burst stimuli and to synthetic speech are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Optical measurement of picometer displacements of transparent microscopic objects

An optical laser differential interferometer, based on a modified differential interference contrast microscope, has been developed to measure the thermal motion of microscopic protrusions (stereocilia) that are the sites of mechanoelectrical transduction in auditory hair cells. The measurement sensitivity was limited at high frequencies mainly by shot noise, at intermediate frequencies by acoustic interference, and at low frequencies by thermal drift. The power spectral density of the instrumental noise was found to be as low as 1. pm/ radicalHz in the shot noise regime. We could, thus, measure the Brownian motion of hair bundles over the frequency range from 1 Hz-100 kHz. Experimental data that test and demonstrate the sensitivity and spatial discrimination of the instrument were found to be in agreement with theoretical estimates.

Saturation of outer hair cell receptor currents causes two-tone suppression

Zwicker [Biol. Cybern. 35, 243–250, (1979); J. Acoust. Soc. Am. 80, 163–176 (1986)] has previously proposed that many nonlinear phenomena in the mammalian cochlea can be explained by saturation of a positive feedback process which enhances mechanical sensitivity, although the site of the nonlinearity producing this saturation has so far remained obscure. In this paper we present evidence suggesting that the nonlinearity of mechano-electrical transduction in the outer hair cells is the dominant nonlinearity producing two-tone suppression in the mammalian cochlea. In particular, we show that: 1. (i) suppression of the extracellular summating potential (SP), recorded from a particular place within the organ of Corti, has characteristics similar to the suppression of activity in the auditory-nerve; 2. (ii) that SP suppression occurs at approximately constant basilar membrane displacement, inferred from the SP iso-response contours; and that 3. (iii) the onset of SP suppression with suppressor tones on the tail of the frequency tuning curve closely parallels the onset of nonlinearity in the local cochlear microphonic. Since previous studies (Patuzzi et al., 1989) have demonstrated that the vibration of the basilar membrane at its characteristic frequency is very sensitive to changes in outer hair cell receptor current, we consider that interference in outer hair cell currents caused by nonlinearity in mechano-electrical transduction is an adequate explanation of two-tone suppression. This requires that outer hair cell receptor currents deviate from linearity at a suppressor tone level below that required to produce a significant DC receptor potential within the inner hair cells, and that the active process within the cochlea is distributed along a local region of the cochlea, basal of the vibration peak.

Ultrastructural Correlates of Mechanoelectrical Transduction in Hair Cells of the Bullfrog's Internal Ear

Ultrastructural Correlates of Mechanoelectrical Transduction in Hair Cells of the Bullfrog's Internal Ear